This invention pertains to an extruder screw for use in the barrel of a plastics material extruder, and is more particularly directed to the construction and design of the metering section of such a screw.
Typically, the metering section of the screw immediately follows a section in which the plastic material has been fully melted. Often, this section is called the "pressure section" since it is the region in which the molten material is stabilized in temperature, is further mixed, and is delivered under pressure, usually to a final mixing or kneading stage before exiting the barrel.
Mixing is an important task of a single screw extruder, the other tasks being conveying and melting. However,, mixing in such single extruders has always been a problem due to lack of uniformity and homogeneity in the melt.
The typical single flight single screw extruder inherently produces a non-uniform mix. This is because there is a large difference in the shear rate and the residence time in the outer and inner regions of the channel between the flighting. These result in a variation in the shear strain within the channel. The circular flow, both parallel to and normal to the screw, results in a shearing action in the outer portion which is counteracted by a shearing action in the inner portion, with locations in which the shear is zero. Such models lack elongational or extentional mixing and distributing mixing mechanisms, and therefore, complete mixing must depend upon the extent and efficiency of the customary final mixing stage.
Also, screw designs in the metering section have, commonly not adequately provided for a reduction or elimination of pressure fluctuations within this section. Flighting channels which vary in depth such as by tapering the core diameter, can, in some instances, produce a fluctuation in the rate of flow which, in turn, can be seen as a fluctuation at the output end of the screw.